Loads and impacts on steel structures in multi-storey buildings. External and internal loads and impacts on individual structural elements and the building as a whole Loads acting on buildings and structures

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Loads and actions on buildings

Buildings as a whole and their individual parts experience various influences from loads (mechanical forces) and influences, for example, from changes in the temperature of the outdoor and indoor air.

Under the influence of these loads and influences, internal forces arise in the materials of building structures, the magnitude of which, per unit area (intensity of internal forces), is called stress. Stress is most commonly measured in kg/cm2.

As a result of stresses in materials and structures, deformations can occur, i.e., tension, compression, shear, bending, torsion, or more complex deformations.

Deformations can be elastic, i.e., disappearing after the removal of the impact that caused the deformation, and plastic, i.e., remaining after the removal of the impact.

The load can be concentrated when its pressure area is small compared to the size of the body to which it is applied, and can be taken as a point, for example, the load from a person on the floor.

If the pressure area is relatively large, then the load is called distributed. If the load is evenly distributed over the area, then it is called uniformly distributed, for example, the weight of a layer of water on water-filled flat surfaces. The nature of the application of loads may be different, for example, on the wall of the basement of a building from the outside, the pressure of the soil increases as it deepens and is expressed as a triangle with a base at the level of the basement floor.

The tensile strength, or tensile strength of the material, is the stress in the material under various types of deformation (tension, compression, torsion, bending), corresponding to the maximum (before the destruction of the sample) load value, and is measured by the ratio of the maximum load to the area of ​​the initial section of the sample (i.e., e. cross sections of the undeformed sample) usually in kg/cm2.

The main characteristics of the resistance of materials to force impacts are the standard resistances (R”), established on the basis of tests.

Rice. 1. Scheme of distribution of loads in the building
a - plan; b - cut

Regulatory resistances can be mainly tensile strengths for various deformations or yield strengths of materials, which are stresses for various types of deformation, which are characterized by the fact that the residual (plastic) deformation is distributed over the entire working volume of the sample at a constant acting load. The normative resistances of various materials and structures are given in SNiP II-A. 10-62.

A possible change in the resistance of materials, products and structures in an unfavorable direction compared to the normative ones, caused by the variability of mechanical properties (heterogeneity of materials), is taken into account by the coefficients of uniformity (k), which are given in SNiP II-A 10-62.

Features of the work of materials, structural elements and their connections, bases, as well as structures and buildings as a whole, not directly reflected in the calculations, are taken into account by the coefficients of working conditions (t) given in SNiP II-A. 10-62.

The resistances of materials taken into account by the calculation are called design resistances ® and are defined as the product of standard resistances (R1 ') by uniformity coefficients (/g), and, if necessary, by working conditions coefficients (t).

The values ​​of design resistances for determining the calculation conditions, taking into account the relevant coefficients of working conditions, are established by the design standards for building structures and foundations of buildings and structures for various purposes.

The greatest loads and impacts that do not restrict or violate normal operating conditions and, if possible, are controlled during operation and in production are called normative.

Possible deviation of loads in an unfavorable (greater or smaller) direction from their standard values ​​due to variability of loads or deviations from normal operation conditions is taken into account by overload factors (n) established taking into account the purpose of buildings and structures and their operating conditions.

Various standard loads on ceilings, loads from technological equipment, overhead cranes, snow and wind loads, as well as overload factors are given in chapter SNiP II-A. 11-62.

The loads taken into account by the calculation, defined as the product of standard loads and the corresponding overload factors, are called design loads.

All loads and impacts that cause forces (stresses) in the structures and foundations of structures, taken into account in the design, are divided into permanent and temporary. Permanent loads include such loads and impacts that can take place during the construction or operation of structures constantly, for example: the weight of permanent parts of buildings, the weight and pressure of soils, prestressing forces, the weight of wires on power transmission line supports and antenna devices of communication structures, etc.

Temporary loads are called such loads or impacts that may be absent during certain periods of construction and operation of the structure.

Depending on the duration of action, temporary loads and impacts are divided into:

a) temporary long-acting, which can be observed during the construction and operation of the facility for a long time, for example: loads in the premises of book storages and libraries, loads on the floors of warehouses, weight of stationary equipment, pressure of liquids and gases in tanks and pipelines, etc .;

b) short-term, which can be observed during the construction and operation of the structure for only a short time, for example: loads from mobile handling equipment, snow and wind loads, wave and ice pressures, temperature climatic effects, etc .; »

c) special, the occurrence of which is possible in exceptional cases, for example: seismic effects in areas subject to earthquakes, water pressure during catastrophic floods, loads arising from the destruction of part of the building, etc.

When calculating building structures, not all loads and impacts that affect them are taken into account, but only certain combinations of loads and impacts (basic, additional, special combinations), which are given in SNiP II-A. 10-62 and II-A. 11-62.

According to the nature of the action, the loads are divided into static (changing gradually) and dynamic (impact, rapidly and periodically changing).

Dynamic loads and impacts on building structures are taken into account in accordance with the instructions of regulatory documents for the design and calculation of load-bearing structures subjected to dynamic loads and impacts. In the absence of the data necessary for this, the dynamic effect on structures can be taken into account by multiplying the design loads by the dynamic factors.

During the design, it is necessary to take into account everything that the building must resist in order not to lose its operational and strength qualities. Loads are considered to be external mechanical forces acting on the building, and influences are internal phenomena. To clarify the issue, we classify all loads and impacts according to the following criteria.

By duration:

• constants - the own mass of the structure, the mass and pressure of the soil in embankments or backfills;
• long-term - the mass of equipment, partitions, furniture, people, snow load, this also includes impacts due to shrinkage and creep of building materials;
• short-term - temperature, wind and ice climatic effects, as well as those associated with changes in humidity, solar radiation;
• special - normalized loads and impacts (for example, seismic, when exposed to fire, etc.).

Among designers, there is also the term payload, the meaning of which is not fixed in regulatory documents, but the term exists in construction practice. The payload is the sum of some temporary loads that are always present in the building: people, furniture, equipment. For example, for a residential building it is 150 ... 200 kg / m 2 (1.5 ... 2 MPa), and for an office building - 300 ... 600 kg / m 2 (3 ... 6 MPa).

By nature of work:

• static - dead weight of the structure, snow cover, equipment;
• dynamic - vibration, gust of wind.

According to the place of application of efforts:

• concentrated - equipment, furniture;
• evenly distributed - the mass of the structure, snow cover.

By the nature of the impact:

• loads of a power nature (mechanical) are loads that cause reactive forces; these loads include all the above examples;
• non-forced impacts:
• changes in outdoor air temperatures, which causes linear temperature deformations of building structures;
• flows of vaporous moisture from the premises - affect the material of external fences;
• atmospheric and ground moisture, chemically aggressive environmental impact;
• solar radiation;
• electromagnetic radiation, noise, etc., affecting human health.

All loads of a power nature are included in engineering calculations. The influence of non-forced impacts is also necessarily taken into account in the design. Let's see, for example, how the effect of temperature affects the structure. The fact is that under the influence of temperature, the structure tends to shrink or expand, i.e. change in size. This is prevented by other constructions with which this construction is associated. Consequently, in those places where structures interact, there are reactive forces that need to be perceived. Also in long buildings it is necessary to provide gaps.

Other influences are also subject to calculations: vapor permeability calculation, thermal engineering calculation, etc.

In the process of construction and operation, the building experiences the action of various loads. External influences can be divided into two types: power and non-power or environmental influences.

To power impacts include different types of loads:

permanent- from the own weight (mass) of the elements of the building, the pressure of the soil on its underground elements;

temporary (long term)- from the weight of stationary equipment, long-term stored goods, own weight of permanent elements of the building (for example, partitions);

short-term- from the weight (mass) of mobile equipment (for example, cranes in industrial buildings), people, furniture, snow, from the action of wind;

special– from seismic impacts, impacts as a result of equipment failures, etc.

To non-coercive relate:

temperature impact, causing changes in the linear dimensions of materials and structures, which in turn leads to the occurrence of force effects, as well as affecting the thermal regime of the room;

exposure to atmospheric and ground moisture, as well as vaporous moisture, contained in the atmosphere and in the air of the premises, causing a change in the properties of the materials from which the building structures are made;

air movement causing not only loads (with wind), but also its penetration into the structure and premises, changing their humidity and thermal conditions;

exposure to radiant energy the sun (solar radiation) causing, as a result of local heating, a change in the physical and technical properties of the surface layers of the material, structures, a change in the light and thermal regime of the premises;

exposure to aggressive chemicals contained in the air, which in the presence of moisture can lead to the destruction of the material of the building structures (corrosion phenomenon);

biological effects caused by microorganisms or insects, leading to the destruction of structures made of organic building materials;

exposure to sound energy(noise) and vibration from sources inside or outside the building.

Place of effort loads divided into concentrated(e.g. equipment weight) and equalsmeasuredlydistributed(own weight, snow).

According to the nature of the load, it can be static, i.e. constant in magnitude over time and dynamic(drums).

In direction - horizontal (wind pressure) and vertical (dead weight).

That. the building is subject to a variety of loads in magnitude, direction, nature of action and place of application.

Rice. 2.3. Loads and impacts on the building.

It may turn out such a combination of loads, in which all of them will act in the same direction, reinforcing each other. It is on such unfavorable combinations of loads that building structures rely. The normative values ​​of all efforts acting on the building are given in DBN or SNiP.

It should be remembered that impacts on structures begin from the moment of their manufacture, continue during transportation, during the construction of the building and its operation.

In the process of construction and operation, the building experiences the action of various loads. External influences can be divided into two types: power and non-power or environmental influences.

To power impacts include different types of loads:

permanent- from the own weight (mass) of the elements of the building, the pressure of the soil on its underground elements;

temporary (long term)- from the weight of stationary equipment, long-term stored goods, own weight of permanent elements of the building (for example, partitions);

short-term- from the weight (mass) of mobile equipment (for example, cranes in industrial buildings), people, furniture, snow, from the action of wind;

special– from seismic impacts, impacts as a result of equipment failures, etc.

To non-coercive relate:

temperature effects, causing changes in the linear dimensions of materials and structures, which in turn leads to the occurrence of force effects, as well as affecting the thermal regime of the room;

exposure to atmospheric and ground moisture, as well as vaporous moisture, contained in the atmosphere and in the air of the premises, causing a change in the properties of the materials from which the building structures are made;

air movement causing not only loads (with wind), but also its penetration into the structure and premises, changing their humidity and thermal conditions;

exposure to radiant energy the sun (solar radiation) causing, as a result of local heating, a change in the physical and technical properties of the surface layers of the material, structures, a change in the light and thermal regime of the premises;

exposure to aggressive chemicals contained in the air, which in the presence of moisture can lead to the destruction of the material of the building structures (corrosion phenomenon);

biological effects caused by microorganisms or insects, leading to the destruction of structures made of organic building materials;

exposure to sound energy(noise) and vibration from sources inside or outside the building.

Place of effort loads divided into concentrated(e.g. equipment weight) and evenly distributed(own weight, snow).

According to the nature of the load, it can be static, i.e. constant in magnitude over time and dynamic(drums).

In direction - horizontal (wind pressure) and vertical (dead weight).

That. the building is subject to a variety of loads in magnitude, direction, nature of action and place of application.

Rice. 2.3. Loads and impacts on the building.

It may turn out such a combination of loads, in which all of them will act in the same direction, reinforcing each other. It is on such unfavorable combinations of loads that building structures rely. The normative values ​​of all efforts acting on the building are given in DBN or SNiP.

It should be remembered that impacts on structures begin from the moment of their manufacture, continue during transportation, during the construction of the building and its operation.

4. Basic requirements for buildings and their elements.

Buildings form a material-spatial environment for people to carry out various social processes of life, work and recreation. Therefore, they must meet a number of requirements, basic of them:

– functional(or technological) expediency, i.e. the building must be convenient for work, recreation or other process for which it is intended;

– technical expediency, i.e. buildings must be strong, stable, durable, reliably protect people and equipment from harmful atmospheric influences, and meet fire safety requirements;

– architectural and artistic expressiveness, i.e. it should be attractive in its appearance, favorably affect the psychological state and consciousness of people;

– economic expediency, providing for the minimum cost of construction and operation of the building to obtain the maximum usable area.

– environmental.

Main in a building or room is its functional appointment.

The implementation of one or another function is always accompanied by the implementation of some other function that has an auxiliary character. For example, training sessions in the classroom represent the main function of this room, while the movement of people when filling the classroom and after the end of classes is an auxiliary function. Therefore, one can distinguish main and ancillary functions. The main function for a particular room in another room can be auxiliary, and vice versa.

room- the main structural element or part of the building. Compliance of the premises with one or another function is achieved only when optimal conditions for a person are created in it, i.e. environment corresponding to the function performed by it in the room.

Environmental quality depends on a number of factors. These include:

space necessary for human activities, equipment placement and movement of people;

condition air environment(microclimate) - a supply of air for breathing with optimal parameters of temperature, humidity and speed of its movement. The state of the air environment is also characterized by the degree of air purity, i.e. the amount of impurities harmful to humans (gases, dust);

sound mode - the conditions of audibility in the room (speech, music, signals) corresponding to its functional purpose, and protection from interfering sounds (noise) arising both in the room itself and penetrating from the outside, and having a harmful effect on the human body and psyche;

light mode - the operating conditions of the organs of vision, corresponding to the functional purpose of the room, determined by the degree of illumination of the room;

visibility and visual perception- conditions for the work of people associated with the need to see flat or three-dimensional objects in the room.

The technical feasibility of a building is determined by the solution of its structures, which must be in full compliance with the laws of mechanics, physics, and chemistry.

In accordance with the impact of the environment, a set of technical requirements is imposed on the building and its structures.

Strength- the ability of the building as a whole and its individual structures to perceive external loads and impacts without destruction and significant residual deformations.

Stability (rigidity)- the ability of the building to maintain static and dynamic balance under external influences of the building, depending on the appropriate placement of structures in accordance with the magnitude and direction of the loads and on the strength of their interfaces.

Durability, meaning the strength, stability and safety of the building and its elements over time. It depends on:

creep materials, i.e. from the process of small continuous deformations occurring in materials under conditions of prolonged exposure to loads.

frost resistance materials, i.e. on the ability of the wet material to withstand repeated alternating freezing and thawing;

moisture resistance materials, i.e. their ability to withstand the destructive action of moisture (softening, swelling, warping, delamination, cracking, etc.);

corrosion resistance, those. on the ability of a material to resist destruction caused by chemical and electrical processes;

biostability, those. on the ability of organic building materials to resist the action of insects and microorganisms.

Durability is determined by the maximum service life of buildings. Practical engineering methods for calculating the durability of buildings have not yet been created, therefore, in building codes and building rules by durability conditionally divided into three degrees:

1st degree - service life of more than 100 years;

2nd degree - service life from 50 to 100 years;

3rd degree - service life from 20 to 50 years.

What are the classes of responsibility or the complexity category of an object?
According to DBN V.1.2-14-2009 "General principles for ensuring the reliability and structural safety of buildings, structures, building structures and foundations" and DBN A.2.2-3:2012 "Composition and content of design documentation for construction", which apply to:
- construction objects (buildings and structures) for various purposes.
- component parts of objects, their foundations and structures made of various materials.

CLASSIFICATION OF BUILDING OBJECTS
The classes of consequences (responsibility) of buildings and structures are determined by the level of possible material losses and (or) social losses associated with the termination of operation or the loss of the integrity of the object.

The possible social costs of abandonment should be assessed depending on risk factors such as:
- danger to human health and life;
- a sharp deterioration in the environmental situation in the area adjacent to the facility (for example, in the event of the destruction of storage facilities for toxic liquids or gases, failure of sewage treatment facilities, etc.);
- loss of historical and cultural monuments or other spiritual values ​​of society;
- termination of functioning of communication systems and networks, power supply, transport or other elements of the life support of the population or the security of society;
- the inability to organize the provision of assistance to victims of accidents and natural disasters;
- a threat to the country's defense capability.

COMPLEXITY CATEGORY OF THE CONSTRUCTION OBJECT
The category of complexity of the construction object is determined on the basis of the class of consequences (responsibility) in accordance with the table
Possible economic losses should be assessed by the costs associated both with the need to restore the object that failed, and indirect damage (losses from stopping production, lost profits, etc.).

During the construction process and during operation, the building experiences various loads. The structural material itself resists these forces, and internal stresses arise in it. The behavior of building materials and structures under the influence of external forces and loads is studied by building mechanics.

Some of these forces act on the building continuously and are called constant loads, others - only in separate periods of time and are called temporary loads.

Permanent loads include dead weight of the building, which mainly consists of the weight of the structural elements that make up its supporting frame. Self-weight acts constantly in time and from top to bottom. Naturally, the stresses in the material of the supporting structures in the lower part of the building will always be greater than in the upper part. Ultimately, the entire effect of its own weight is transferred to the foundation, and through it to the foundation soil. Self weight has always been not only constant, but also the main, main load on the building.

Only in recent years, builders and designers have faced a completely new problem: not how to securely support the building on the ground, but how to “tie” it, anchor it to the ground so that it is not torn off the ground by other influences, mainly wind forces. This happened because the own weight of structures as a result of the use of new high-strength materials and new design schemes was constantly decreasing, and the dimensions of buildings were growing. The area on which the wind acts, in other words, the windage of the building, increased. And finally, the effect of the wind became more "weighty" than the effect of the weight of the building, and the building began to tend to lift off the ground.

is one of the main temporary loads. As altitude increases, the effect of wind increases. So, in the middle part of Russia, the load from the wind (wind speed) at a height of up to 10 m is assumed to be 270 Pa, and at a height of 100 m it is already equal to 570 Pa. In mountainous areas, on the sea coasts, the impact of the wind is much greater. For example, in some areas of the coastline of the Arctic and Primorye, the standard value of the wind pressure at a height of up to 10 m is 1 kPa. A rarefied space appears on the leeward side of the building, which creates a negative pressure - suction, which increases the overall effect of the wind. The wind changes both direction and speed. Strong gusts of wind also create a shock, dynamic effect on the building, which further complicates the conditions for the operation of the structure.

Urban planners faced big surprises when they began to erect high-rise buildings in cities. It turned out that the street, which never had strong winds, became very windy with the construction of multi-storey buildings on it. From the point of view of a pedestrian, the wind at a speed of 5 m / s is already becoming annoying: it blows clothes, spoils the hair. If the speed is a little higher - the wind is already raising dust, swirling scraps of paper, it becomes unpleasant. A tall building is a solid barrier to air movement. Hitting this barrier, the wind breaks into several streams. Some of them go around the building, others rush down, and then near the ground they also go to the corners of the building, where the strongest air flows are observed, 2-3 times faster than the wind that would blow in this place if there were no building. With very tall buildings, the force of the wind at the base of the building can be of such magnitude that it knocks down pedestrians.

The amplitude of oscillations of high-rise buildings reaches large sizes, which negatively affects the well-being of people. The creaking and sometimes creaking of the steel frame of one of the world's tallest buildings of the International Trade Center in New York (its height is 400 m) causes an alarming condition for people in the building. It is very difficult to foresee and calculate in advance the effect of wind in high-rise construction. Currently, builders resort to experiments in a wind tunnel. Just like the aircraft builders! they blow models of future buildings in it and to some extent get a real picture of air currents and their strength.

also applies to live loads. Particular attention should be paid to the effect of snow load on buildings of different heights. On the border between the elevated and lowered parts of the building, a so-called "snow bag" appears, where the wind collects entire snowdrifts. At variable temperatures, when the snow alternately thaws and freezes again, and at the same time suspended particles from the air (dust, soot) get here, snow, or rather, ice massifs become especially heavy and dangerous. Due to the wind, the snow cover falls unevenly both with flat and pitched roofs, creating an asymmetric load, which causes additional stresses in the structures.

Temporary includes (load from people who will be in the building, process equipment, stored materials, etc.).

Stresses arise in the building from exposure to solar heat and frost. This effect is called temperature and climate. Being heated by the sun's rays, building structures increase their volume and size. Cooling during frosts, they decrease in volume. With such a “breathing” of the building, stresses arise in its structures. If the building has a large extent, these stresses can reach high values, exceeding the allowable ones, and the building will begin to collapse.

Similar stresses in the material of construction also arise when uneven settlement of the building, which can occur not only because of the different bearing capacity of the base, but also because of the large difference in payload or dead weight of individual parts of the building. For example, a building has multi-story and one-story parts. In the multi-storey part, heavy equipment is located on the floors. The pressure on the ground from the foundations of a multi-story part will be much greater than from the foundations of a one-story part, which can cause uneven settlement of the building. To remove additional stresses from sedimentary and temperature effects, the building is “cut” into separate compartments with expansion joints.

If the building is protected from temperature deformations, then the seam is called temperature. It separates the structures of one part of the building from another, with the exception of foundations, since foundations, being in the ground, do not experience temperature effects. Thus, the thermal seam localizes additional stresses within one compartment, preventing their transfer to neighboring compartments, thereby preventing their addition and increase.

If the building is protected from sedimentary deformations, then the seam is called sedimentary. It separates one part of the building from the other completely, including the foundations, which, thanks to such a seam, are able to move one in relation to the other in a vertical plane. In the absence of seams, cracks could appear in unexpected places and compromise the strength of the building.

In addition to permanent and temporary, there are also special effects on buildings. These include:

• seismic loads from an earthquake;
• explosive impacts;
• loads arising from accidents or breakdowns of process equipment;
• impacts from uneven deformations of the base during soaking of subsiding soils, during thawing of permafrost soils, in areas of mine workings and during karst phenomena.

According to the place of application of efforts, loads are divided into concentrated (for example, the weight of equipment) and evenly distributed (own weight, snow, etc.).

By the nature of the action, loads can be static, that is, constant in magnitude over time, for example, the same own weight of structures, and dynamic (impact), for example, gusts of wind or the impact of moving parts of equipment (hammers, motors, etc.).

Thus, a variety of loads act on the building in terms of magnitude, direction, nature of action and place of application (Fig. 5). You can get a combination of loads in which they all act in the same direction, reinforcing each other.

Rice. 5. Loads and impacts on the building: 1 - wind; 2 - solar radiation; 3 - precipitation (rain, snow); 4 - atmospheric influences (temperature, humidity, chemicals); 5 - payload and own weight; 6 - special effects; 7 - vibration; 8 - moisture; 9 - soil pressure; 10 - noise

It is on such unfavorable combinations of loads that building structures rely. The normative values ​​of all efforts acting on the building are given in SNiP. It should be remembered that impacts on structures begin from the moment of their manufacture, continue during transportation, during the construction of the building and its operation.

Blagoveshchensky F.A., Bukina E.F. Architectural designs. - M., 1985.